Transition between planes
Go to Edit - Preferences... Add-ons tab and search for LoopTools. Enable the add-on.
Select both objects and press CTRL  + J to join them.
hold ALT and click right to the right edge of the left plane. THen hold SHIFT + ALT and click left to the left edge of the right plane.
Right click and choose LoopTools -&gt; Bridge



Press CTRL + R, hover over the bridge and use your mouse wheel to get  a uniform density of loops across whole mesh.